After studying this chapter, you will be able to:
❍ Explain the purpose of datums.
❍ Defi ne common terms related to datum features.
❍ Interpret information provided by datum feature symbols.
❍ Describe methods for representing datum surfaces on drawings.
❍ Explain how to establish datum reference frames.
❍ Identify the degrees of freedom of a part.
❍ Explain how to establish datum targets.
❍ Identify datum target points, areas, and lines.
❍ Describe methods for representing datum axes on drawings.
❍ Describe methods for representing datum center planes on drawings.
Actual mating envelope Normal
Coaxial Order of precedence
Coaxial datum features Primary datum
Coplanar surfaces Primary datum plane
Datum axis Rotational
Datum feature Secondary datum
Datum feature simulator Secondary datum plane
Datum feature symbol Simulated datum
Datum plane Simulated datum axis
Datum reference frame (DRF) Simulated datum plane
Datum reference order Tangent plane
Datum target line Tertiary datum
Datum target point Tertiary datum plane
Datum targets Three degrees of rotation
Datums Three degrees of
Degrees of freedom translation
High points Translational
Movable datum targets
Datums are considered to be theoretically perfect planes, surfaces, points, lines,
axes or a combination thereof derived from the datum feature simulator, which is
described in this chapter.